A condensation particle counter comprises a saturator, a condenser and an optical particle counter (OPC) and is widely used in measuring the number and size of fine particles. The saturator of the condensation particle counter contains working fluid to saturate an aerosol, i.e., a particle-suspended gas. Examples of the working fluid include alcohol-based fluid such as alcohol, butanol, isopropyl alcohol or the like and organic compound fluid such as ethylene glycol or the like.
The saturator is heated by a heater and kept at a temperature higher than an ambient temperature, as a result of which the working fluid contained in the saturator is evaporated. If the particle-suspended gas is introduced into the saturator, it is saturated by the working fluid and turned to a saturated gas which in turn is supplied to the condenser from the saturator. In case of using alcohol as the working fluid, the saturator is kept at a temperature of 35° C. and the condenser is kept at a temperature of 10° C. As the temperature of the condenser drops, the saturated gas is turned to a supersaturated gas. Condensation of the saturated gas occurs in such a fashion that liquid droplets grow bigger around fine particles as their nuclei. The liquid droplets thus grown are supplied to the optical particle counter. If the saturated gas is diffused faster than the heat transfer speed in the condenser, the saturated gas is not condensed around the fine particles but condensed only on the wall surface of the condenser. The optical particle counter is designed to count the number and size of the fine particles by detecting the liquid droplets.
There are provided many advantages if water is used as the working fluid of the condensation particle counter. This is because water is not harmful to the human body and does not generate any odor or pollutant. In case water is used as the working fluid in the conventional condensation particle counter, however, the water vapor is condensed only on the wall surface of the condenser that remains at a low temperature and there occurs no condensation that uses fine particles as nuclei. Therefore, the fine particles are discharged as they are. This poses a problem in that the fine particles cannot be detected by means of the optical particle counter.
For the reasons mentioned above, the conventional condensation particle counter makes use of an organic compound as the working fluid. The organic compound is detrimental to the human body and gives off a strong smell. In addition, the organic compound is highly difficult to handle because it is flammable. Particularly, if an alcohol-based organic material is used as the working fluid of the condensation particle counter in a semiconductor manufacturing process, the organic material acts as a pollutant and therefore becomes a cause of defect. Inasmuch as the organic compound tends to absorb moisture contained in a gas, the condensation particle counter suffers from degradation in performance if the organic compound is used for a long period of time. Thus, there is a need to periodically replace the organic compound, which task is onerous.
The conventional condensation particle counter has a fixed aerosol measuring capacity. Therefore, several condensation particle counters have to be used in combination in order to measure a large quantity of aerosol in one place, which is cumbersome and inconvenient. Furthermore, in case of using several condensation particle counters at one time, it is very difficult to uniformly control the temperature of the saturator and the condenser of the individual condensation particle counters. If a great deviation exists in the temperature of the saturator and the condenser, the data obtained by measuring the fine particles become less reliable.